Understanding how viruses disrupt normal cellular processes has provided tremendous insights into molecular mechanisms that underlie host cell function. Infection by mammalian reovirus, a highly tractable model for studies of dsRNA virus replication and pathogenesis, profoundly alters host cell physiology leading to cell cycle arrest and induction of apoptosis, processes intricately involved in cancer, immunity, and development. The reovirus nonstructural protein, a1s, is implicated in perturbation of the cell cycle and induction of apoptosis. However, mechanisms by which a1s mediates these effects are not known. Using a newly developed, plasmid-based reverse genetics system for reovirus, viruses were generated that contain either the wild-type a1s-encoding S1 gene segment or an S1 gene segment in which the a1s translational start site has been altered to block a1s synthesis. Experiments described in this application will use these otherwise isogenic viruses to define the function of a1s in reovirus replication and pathogenesis and determine the role of a1s in reovirus-induced cell cycle arrest and apoptosis. Three specific aims are proposed. The first specific aim will define the function of a1s in reovirus replication in cultured cells and determine whether a1s contributes to reovirus-induced apoptosis. The second specific aim will determine the effects of a1s on cell cycle progression and use proteomic approaches to identify cellular proteins with which a1s interacts. The third specific aim will define the contribution of a1s to reovirus-induced pathogenesis in vivo. Collectively, these studies will provide new insights into mechanisms by which reovirus infection modulates the host cell cycle and induces apoptosis. The goal of the experiments described in this application is to determine the function of reovirus a1s protein in modulating cell cycle progression, apoptosis, and viral pathogenesis. Findings from these studies could lead to new insights into these highly-regulated, interrelated processes that play fundamental roles in development, immunity, and cancer. Such insights might lead to new treatments for degenerative, immunologic, or neoplastic diseases. [unreadable] [unreadable] [unreadable]